In an Institute of Electrical and Electronics Engineers (IEEE) 802.16e system, a Hybrid Automatic Retransmit Request (HARQ) operation is performed in frame units, acknowledgement/negative acknowledgement (ACK/NACK) delay for a HARQ burst is set to one value, and ACK/NACK delay is transmitted through a Downlink Channel Descriptor (DCD) or an Uplink Channel Descriptor (UCD).
In contrast, in an IEEE 802.16m system, one frame may be divided into n subframes and a HARQ operation may be performed in subframe units. Delay which is minimally required by a mobile station (MS) and a base station (BS) occurs when the HARQ operation is performed.
First, delay associated with a downlink HARQ operation will be described with reference to FIG. 1. FIG. 1 is a diagram showing a downlink HARQ operation.
In FIG. 1, DDL—a denotes a minimal time required for transmitting ACK/NACK for a downlink HARQ burst to a BS after an MS receives the downlink HARQ burst from the BS, and DDL—b denotes a minimal time required for transmitting new data or retransmitting a downlink HARQ burst after the BS receives ACK or NACK from the MS.
That is, in the downlink HARQ operation, the MS receives the downlink HARQ burst from the BS and then transmits ACK/NACK to the BS through an uplink subframe after DDL—a. The BS receives ACK/NACK, transmits new data if ACK is received through a subframe after DDL—b, and retransmits the downlink HARQ burst if NACK is received.
Next, delay associated with an uplink HARQ operation will be described with reference to FIG. 2. FIG. 2 is a diagram showing the uplink HARQ operation.
In FIG. 2, DUL—a denotes a minimal time required for generating and transmitting a HARQ burst after an MS receives an uplink A-MAP, DUL—b denotes a minimal time required for transmitting ACK/NACK for a HARQ burst after the BS receives the HARQ burst from the MS, DUL—c denotes a minimal time required for transmitting new data or retransmitting a downlink HARQ burst after the MS receives ACK or NACK from the BS. At this time, DUL—a and DUL—c may have the same value.
The HARQ operation is basically performed with respect to one HARQ channel in a stop-and-wait manner. That is, until ACK for transmission is received, new transmission is not performed but retransmission is performed.
In addition, minimal delay occurs when transmitting a HARQ burst, receiving ACK/NACK and transmitting new data or retransmitting HARQ burst with respect to one HARQ channel. In order to compensate for such delay, multiple HARQ channels are allocated and used to transmit data. However, if the number of HARQ channels is increased, the total delay of one HARQ channel is increased and delay for successfully transmitting one HARQ burst is increased. Therefore, system performance is deteriorated.
Accordingly, the number of HARQ channels is preferably as small as possible in consideration of delay associated with the HARQ operation. That is, a process of transmitting a HARQ burst, receiving ACK/NACK and transmitting new data or retransmitting HARQ burst with respect to one HARQ channel is performed as soon as possible.
If one frame is divided into a plurality of subframes and a HARQ operation is performed in subframe units, HARQ ACK delay does not have one value as in the IEEE 802.16e system but has different values according to subframes.
For example, ACK/NACK for a downlink HARQ burst transmitted in a b-th downlink subframe of an a-th frame may be transmitted in a c-th uplink subframe of an a-th frame, but ACK/NACK for a downlink HARQ burst transmitted in a (b+1)-th downlink subframe may be transmitted in a d-th uplink subframe of an (a+1)-th frame. Such a situation may occur according to an ACK delay value. Since such a situation occurs, if ACK delay is explicitly set as in the method of the IEEE 802.16e system, significant signaling overhead occurs.